The primary coil is the internal heat exchanger through which water from the heat pump flows to heat the stored water in the cylinder. This is a critical specification for heat pump use, and it’s where many standard cylinders fail to perform.
There are two main types of hot water cylinder:
Unvented cylinders are connected directly to the mains cold water supply. This means hot water is delivered throughout the home at mains pressure — no cold tank in the loft, no gravity-fed low-pressure taps, no need for a shower pump. Hot water pressure is consistent, powerful, and equal at all outlets. This is the standard specification for most heat pump installations in 2026, and is recommended for new installations in most homes.
Unvented systems require installation by a Part L2 (or G3) qualified plumber — this is a legal requirement because the system contains water under mains pressure and requires specific safety devices (pressure relief valve, expansion vessel, tundish). Most heat pump installers are appropriately qualified, but confirm this before signing off a quote.
Vented cylinders are fed from a cold water storage tank in the loft and deliver hot water at low gravity pressure. They are simpler and cheaper, and remain appropriate in some scenarios — particularly older properties where the existing plumbing is already gravity-fed and a full replumb is not practical or economic. However, low pressure hot water is increasingly unsatisfactory for modern households, and the cold water loft tank is an additional maintenance consideration. For any new installation where a free choice exists, unvented is the better option.
Cylinder sizing is based on daily hot water demand — how much hot water your household uses in a typical day. The goal is to store enough hot water for a day’s use without needing to reheat mid-day, while not buying more volume than you need (larger cylinders cost more and take longer to heat). Here are the standard sizing guidelines, which align with Energy Saving Trust recommendations:
| Household size | Recommended cylinder volume | Notes |
|---|---|---|
| 1–2 people | 150–180 litres | A 150L is the minimum practical size for heat pump use |
| 3–4 people | 200–250 litres | Most common requirement for UK families; 210L or 250L is typical |
| 5–6 people | 250–300 litres | Also appropriate for high hot water demand households (frequent baths, multiple showers) |
| 6+ people or high demand | 300 litres+ | Some installations use two cylinders in series; discuss with your installer |
A common mistake is sizing the cylinder too small to save cost on the initial installation. A 150L cylinder in a four-person household that includes teenagers who shower daily will result in the heat pump constantly running reheat cycles, which raises electricity consumption and may cause hot water shortfalls. Sizing up by 50 litres costs £100–£200 more but can save hundreds of pounds in electricity over the system’s lifetime.
The primary coil is the internal heat exchanger through which water from the heat pump flows to heat the stored water in the cylinder. This is a critical specification for heat pump use, and it’s where many standard cylinders fail to perform.
Heat pumps deliver heat at low flow temperatures (40–55°C for domestic hot water). For this to efficiently heat a cylinder of stored water to 50°C, the coil needs a large surface area — larger than is typically found in cylinders designed for gas boiler use. The standard metric is coil surface area in m²:
Using a standard cylinder with a heat pump is a known mistake in the industry. It forces the heat pump to run at higher flow temperatures to achieve the target stored water temperature, reducing efficiency and increasing running costs. Always confirm with your installer that the specified cylinder is heat pump-compatible and has an appropriately sized primary coil. Leading heat pump-optimised cylinder brands include Gledhill, Joule, Telford, and HeatStore.
Legionella bacteria can proliferate in stored water held between 20°C and 45°C. Most heat pumps store domestic hot water at 50–55°C, which is above the danger zone, but as an additional safeguard, UK guidelines recommend a weekly pasteurisation cycle in which the entire cylinder volume is raised to 60°C and held there for at least an hour. This kills any residual bacteria.
At 60°C, the heat pump becomes inefficient (COP drops to around 2.0 or below) — so the legionella cycle is typically run using the immersion heater element that is built into all heat pump cylinders. Your heat pump controller should be configured to run this automatically, typically overnight once per week when electricity prices are lower. This is done during commissioning; confirm with your installer that the legionella cycle is enabled and scheduled. The HSE guidance on Legionella in domestic hot water systems provides the regulatory background.
A heat pump-compatible indirect unvented cylinder costs £500–£1,200 for the unit itself, depending on size and brand. Installation (plumbing labour, pressure safety devices, electrical connection for the immersion heater) adds a further £400–£800. Total cost for a new cylinder as part of a heat pump project: typically £900–£1,800. If your home already has a suitable indirect cylinder in good condition, your installer may be able to reuse it — though confirm it has an adequate primary coil surface area before assuming compatibility.
If space for a cylinder is limited (common in homes that have only ever had a combi boiler), your installer will need to identify a suitable cupboard or utility room location. A standard 200L cylinder requires a space of approximately 65 cm diameter and 120–140 cm height. Some properties may need a slimline model or a rearrangement of storage space. This is worth discussing early in the survey stage — it’s rarely an insurmountable issue, but it’s better to plan for it than to discover the problem on installation day.
For the full picture on installation costs, including how the cylinder cost fits into the total project budget, see our heat pump costs guide for 2026. And for an overview of the complete installation process from survey to switch-on, see our heat pump installation guide. If you’re assessing whether the whole project makes financial sense, start with is a heat pump worth it in the UK?
The MCS consumer guidance on heat pump installation and the government’s BUS grant page are the authoritative references for grant-funded installations.
A combi (combination) boiler heats water on demand by burning gas at a high rate, instantly delivering hot water at the tap. This works because gas combustion produces a large amount of heat very quickly — the boiler can raise water temperature to 60°C in seconds. A heat pump cannot do this. It is a slow-heating device that moves heat rather than generating it — heating water efficiently at low temperature over a longer period. There is no practical way to design a heat pump system that heats water on demand the way a combi does.
Instead, a heat pump pre-heats water and stores it in an insulated cylinder, ready to be drawn on at any time. As long as the cylinder is correctly sized for household demand, this works seamlessly — and in practice most households adapt quickly because the cylinder reheat time (typically two to three hours) is fast enough that they never notice it unless they’re using an unusually large amount of hot water in a short period.
If you’re converting from a combi boiler, adding a cylinder is a mandatory part of the heat pump installation. The cost is typically £800–£1,800 for the cylinder and associated plumbing — see our heat pump costs guide for a full breakdown of what this adds to your project.
There are two main types of hot water cylinder:
Unvented cylinders are connected directly to the mains cold water supply. This means hot water is delivered throughout the home at mains pressure — no cold tank in the loft, no gravity-fed low-pressure taps, no need for a shower pump. Hot water pressure is consistent, powerful, and equal at all outlets. This is the standard specification for most heat pump installations in 2026, and is recommended for new installations in most homes.
Unvented systems require installation by a Part L2 (or G3) qualified plumber — this is a legal requirement because the system contains water under mains pressure and requires specific safety devices (pressure relief valve, expansion vessel, tundish). Most heat pump installers are appropriately qualified, but confirm this before signing off a quote.
Vented cylinders are fed from a cold water storage tank in the loft and deliver hot water at low gravity pressure. They are simpler and cheaper, and remain appropriate in some scenarios — particularly older properties where the existing plumbing is already gravity-fed and a full replumb is not practical or economic. However, low pressure hot water is increasingly unsatisfactory for modern households, and the cold water loft tank is an additional maintenance consideration. For any new installation where a free choice exists, unvented is the better option.
Cylinder sizing is based on daily hot water demand — how much hot water your household uses in a typical day. The goal is to store enough hot water for a day’s use without needing to reheat mid-day, while not buying more volume than you need (larger cylinders cost more and take longer to heat). Here are the standard sizing guidelines, which align with Energy Saving Trust recommendations:
| Household size | Recommended cylinder volume | Notes |
|---|---|---|
| 1–2 people | 150–180 litres | A 150L is the minimum practical size for heat pump use |
| 3–4 people | 200–250 litres | Most common requirement for UK families; 210L or 250L is typical |
| 5–6 people | 250–300 litres | Also appropriate for high hot water demand households (frequent baths, multiple showers) |
| 6+ people or high demand | 300 litres+ | Some installations use two cylinders in series; discuss with your installer |
A common mistake is sizing the cylinder too small to save cost on the initial installation. A 150L cylinder in a four-person household that includes teenagers who shower daily will result in the heat pump constantly running reheat cycles, which raises electricity consumption and may cause hot water shortfalls. Sizing up by 50 litres costs £100–£200 more but can save hundreds of pounds in electricity over the system’s lifetime.
The primary coil is the internal heat exchanger through which water from the heat pump flows to heat the stored water in the cylinder. This is a critical specification for heat pump use, and it’s where many standard cylinders fail to perform.
Heat pumps deliver heat at low flow temperatures (40–55°C for domestic hot water). For this to efficiently heat a cylinder of stored water to 50°C, the coil needs a large surface area — larger than is typically found in cylinders designed for gas boiler use. The standard metric is coil surface area in m²:
Using a standard cylinder with a heat pump is a known mistake in the industry. It forces the heat pump to run at higher flow temperatures to achieve the target stored water temperature, reducing efficiency and increasing running costs. Always confirm with your installer that the specified cylinder is heat pump-compatible and has an appropriately sized primary coil. Leading heat pump-optimised cylinder brands include Gledhill, Joule, Telford, and HeatStore.
Legionella bacteria can proliferate in stored water held between 20°C and 45°C. Most heat pumps store domestic hot water at 50–55°C, which is above the danger zone, but as an additional safeguard, UK guidelines recommend a weekly pasteurisation cycle in which the entire cylinder volume is raised to 60°C and held there for at least an hour. This kills any residual bacteria.
At 60°C, the heat pump becomes inefficient (COP drops to around 2.0 or below) — so the legionella cycle is typically run using the immersion heater element that is built into all heat pump cylinders. Your heat pump controller should be configured to run this automatically, typically overnight once per week when electricity prices are lower. This is done during commissioning; confirm with your installer that the legionella cycle is enabled and scheduled. The HSE guidance on Legionella in domestic hot water systems provides the regulatory background.
A heat pump-compatible indirect unvented cylinder costs £500–£1,200 for the unit itself, depending on size and brand. Installation (plumbing labour, pressure safety devices, electrical connection for the immersion heater) adds a further £400–£800. Total cost for a new cylinder as part of a heat pump project: typically £900–£1,800. If your home already has a suitable indirect cylinder in good condition, your installer may be able to reuse it — though confirm it has an adequate primary coil surface area before assuming compatibility.
If space for a cylinder is limited (common in homes that have only ever had a combi boiler), your installer will need to identify a suitable cupboard or utility room location. A standard 200L cylinder requires a space of approximately 65 cm diameter and 120–140 cm height. Some properties may need a slimline model or a rearrangement of storage space. This is worth discussing early in the survey stage — it’s rarely an insurmountable issue, but it’s better to plan for it than to discover the problem on installation day.
For the full picture on installation costs, including how the cylinder cost fits into the total project budget, see our heat pump costs guide for 2026. And for an overview of the complete installation process from survey to switch-on, see our heat pump installation guide. If you’re assessing whether the whole project makes financial sense, start with is a heat pump worth it in the UK?
The MCS consumer guidance on heat pump installation and the government’s BUS grant page are the authoritative references for grant-funded installations.
One of the most common surprises for homeowners switching to a heat pump is finding out that they need a hot water cylinder — especially if they’ve had a combi boiler for years. The reason is fundamental to how a heat pump works: unlike a combi boiler, which heats water on demand to high temperatures in seconds, a heat pump heats water slowly and stores it in an insulated cylinder. Get the cylinder sizing and specification right, and you’ll have reliable hot water and an efficient system. Get it wrong, and you’ll run out of hot water at inconvenient moments or pay more to run the system than you should. This guide covers everything you need to know.
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A combi (combination) boiler heats water on demand by burning gas at a high rate, instantly delivering hot water at the tap. This works because gas combustion produces a large amount of heat very quickly — the boiler can raise water temperature to 60°C in seconds. A heat pump cannot do this. It is a slow-heating device that moves heat rather than generating it — heating water efficiently at low temperature over a longer period. There is no practical way to design a heat pump system that heats water on demand the way a combi does.
Instead, a heat pump pre-heats water and stores it in an insulated cylinder, ready to be drawn on at any time. As long as the cylinder is correctly sized for household demand, this works seamlessly — and in practice most households adapt quickly because the cylinder reheat time (typically two to three hours) is fast enough that they never notice it unless they’re using an unusually large amount of hot water in a short period.
If you’re converting from a combi boiler, adding a cylinder is a mandatory part of the heat pump installation. The cost is typically £800–£1,800 for the cylinder and associated plumbing — see our heat pump costs guide for a full breakdown of what this adds to your project.
There are two main types of hot water cylinder:
Unvented cylinders are connected directly to the mains cold water supply. This means hot water is delivered throughout the home at mains pressure — no cold tank in the loft, no gravity-fed low-pressure taps, no need for a shower pump. Hot water pressure is consistent, powerful, and equal at all outlets. This is the standard specification for most heat pump installations in 2026, and is recommended for new installations in most homes.
Unvented systems require installation by a Part L2 (or G3) qualified plumber — this is a legal requirement because the system contains water under mains pressure and requires specific safety devices (pressure relief valve, expansion vessel, tundish). Most heat pump installers are appropriately qualified, but confirm this before signing off a quote.
Vented cylinders are fed from a cold water storage tank in the loft and deliver hot water at low gravity pressure. They are simpler and cheaper, and remain appropriate in some scenarios — particularly older properties where the existing plumbing is already gravity-fed and a full replumb is not practical or economic. However, low pressure hot water is increasingly unsatisfactory for modern households, and the cold water loft tank is an additional maintenance consideration. For any new installation where a free choice exists, unvented is the better option.
Cylinder sizing is based on daily hot water demand — how much hot water your household uses in a typical day. The goal is to store enough hot water for a day’s use without needing to reheat mid-day, while not buying more volume than you need (larger cylinders cost more and take longer to heat). Here are the standard sizing guidelines, which align with Energy Saving Trust recommendations:
| Household size | Recommended cylinder volume | Notes |
|---|---|---|
| 1–2 people | 150–180 litres | A 150L is the minimum practical size for heat pump use |
| 3–4 people | 200–250 litres | Most common requirement for UK families; 210L or 250L is typical |
| 5–6 people | 250–300 litres | Also appropriate for high hot water demand households (frequent baths, multiple showers) |
| 6+ people or high demand | 300 litres+ | Some installations use two cylinders in series; discuss with your installer |
A common mistake is sizing the cylinder too small to save cost on the initial installation. A 150L cylinder in a four-person household that includes teenagers who shower daily will result in the heat pump constantly running reheat cycles, which raises electricity consumption and may cause hot water shortfalls. Sizing up by 50 litres costs £100–£200 more but can save hundreds of pounds in electricity over the system’s lifetime.
The primary coil is the internal heat exchanger through which water from the heat pump flows to heat the stored water in the cylinder. This is a critical specification for heat pump use, and it’s where many standard cylinders fail to perform.
Heat pumps deliver heat at low flow temperatures (40–55°C for domestic hot water). For this to efficiently heat a cylinder of stored water to 50°C, the coil needs a large surface area — larger than is typically found in cylinders designed for gas boiler use. The standard metric is coil surface area in m²:
Using a standard cylinder with a heat pump is a known mistake in the industry. It forces the heat pump to run at higher flow temperatures to achieve the target stored water temperature, reducing efficiency and increasing running costs. Always confirm with your installer that the specified cylinder is heat pump-compatible and has an appropriately sized primary coil. Leading heat pump-optimised cylinder brands include Gledhill, Joule, Telford, and HeatStore.
Legionella bacteria can proliferate in stored water held between 20°C and 45°C. Most heat pumps store domestic hot water at 50–55°C, which is above the danger zone, but as an additional safeguard, UK guidelines recommend a weekly pasteurisation cycle in which the entire cylinder volume is raised to 60°C and held there for at least an hour. This kills any residual bacteria.
At 60°C, the heat pump becomes inefficient (COP drops to around 2.0 or below) — so the legionella cycle is typically run using the immersion heater element that is built into all heat pump cylinders. Your heat pump controller should be configured to run this automatically, typically overnight once per week when electricity prices are lower. This is done during commissioning; confirm with your installer that the legionella cycle is enabled and scheduled. The HSE guidance on Legionella in domestic hot water systems provides the regulatory background.
A heat pump-compatible indirect unvented cylinder costs £500–£1,200 for the unit itself, depending on size and brand. Installation (plumbing labour, pressure safety devices, electrical connection for the immersion heater) adds a further £400–£800. Total cost for a new cylinder as part of a heat pump project: typically £900–£1,800. If your home already has a suitable indirect cylinder in good condition, your installer may be able to reuse it — though confirm it has an adequate primary coil surface area before assuming compatibility.
If space for a cylinder is limited (common in homes that have only ever had a combi boiler), your installer will need to identify a suitable cupboard or utility room location. A standard 200L cylinder requires a space of approximately 65 cm diameter and 120–140 cm height. Some properties may need a slimline model or a rearrangement of storage space. This is worth discussing early in the survey stage — it’s rarely an insurmountable issue, but it’s better to plan for it than to discover the problem on installation day.
For the full picture on installation costs, including how the cylinder cost fits into the total project budget, see our heat pump costs guide for 2026. And for an overview of the complete installation process from survey to switch-on, see our heat pump installation guide. If you’re assessing whether the whole project makes financial sense, start with is a heat pump worth it in the UK?
The MCS consumer guidance on heat pump installation and the government’s BUS grant page are the authoritative references for grant-funded installations.
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